In recent years, piezoelectric, and electrostrictive ceramics have been used as displacement transducers, precision micropositioners, and for other actuator applications. An important drawback to such devices, however, is the fact that the magnitude of strain in such ceramics is limited to approximately 0.1%. Magnification mechanisms have, therefore, been developed to produce sizeable displacements at low voltages. The two most common types of electroactive ceramic actuators are the multi-layer ceramic actuator with internal electrodes, and the cantilevered bimorph actuator.
A bimorph-type actuator will execute a large bending or "wagging" motion with the application of an AC or DC field. Although such actuators exhibit large displacements (generally on the order of several hundred microns), their generative force and response speeds are not high. Multilayer-actuators exhibit significantly larger generative forces, although their displacement values are limited. For instance, a fifteen millimeter multilayer stack provides a displacement of approximately 10 microns. Such a stack characteristically comprises a lead titanate-lead zirconate (PZT) ceramic or a lead titanate-doped lead magnesium niobate (PMN-PT) type ceramic, having a hundred volts of DC applied. There is a need for an electroactive ceramic actuator to provide sizeable displacements with sufficient force to carry out actuator applications.
In U.S. Pat. No. 4,999,819 the inventors hereof previously described an acoustic transducer, of sandwich construction, that was particularly useful for the transformation of hydrostatic pressures to electrical signals. A pair of metal plates were positioned to sandwich a piezoelectric element, with each plate having a cavity formed adjacent to the piezoelectric element. The plates were bonded to the piezoelectric element to provide a unitary structure. The cavities provided a stress-transforming capability which amplified an incoming compressive stress and converted it to a radial extensional stress in the ceramic. The ceramic was generally poled in the thickness dimension and exhibited d.sub.33, d.sub.31 and d.sub.32 piezoelectric coefficients.
As is known to those skilled in the art, the d.sub.33 coefficient lies in the plane of a ceramic's poling, whereas the d.sub.31 and d.sub.32 coefficients describe the action of the ceramic in a plane that is orthogonal to the direction of poling. In the transducer shown in the '819 patent, the cavities transform most of an incoming stress in the d.sub.33 direction to the d.sub.31 and d.sub.32 directions in the piezoelectric slab. By monitoring the voltage generated across the slab, the resulting pressure wave was sensed. There is no indication in U.S. Pat. No. 4,999,819 of the application or use of the aforedescribed structure for actuation purposes.
Accordingly, it is an object of this invention to provide an improved electroactive, metal-ceramic actuator.
It is another object of this invention to provide a metal-ceramic actuator which exhibits substantially improved actuation distances.
It is another object of this invention to provide a metal-ceramic electroactive actuator of inexpensive design.